Casing and tubing failures are a source of great concern to producers of oil and gas. During the drilling of oil and gas wells, at least a portion of the well is lined with a tubular casing. Drilling fluids are circulated for various purposes through the drill string and back to the surface through the casing. Failure of this casing will allow undesirable leakage of drilling fluids into surrounding formations.
A more serious problem may result from the failure of production tubing. Tubing runs from the producing formation downhole up to the wellhead. If the tubing fails, the gas or oil being produced may leak to the surface along the outside of the tubing, and thence into the atmosphere, creating a risk of explosion or fire. In the case of subsea wells, the leakage may flow into the water causing serious pollution.
Many of the failures of casing and tubing are caused by longitudinal manufacturing flaws, such as laps, seams and plug scores, or from service induced defects such as caliper tracks. As such, it is not unusual for each length of casing or tubing to be inspected prior to installation in a well. Although inspection of the shank portion of tubing and casing is relatively straightforward, the inspection of the end areas of tubing and casing is not as simple. These ends are often "upset" (manufactured so that the wall of the tubular product is of extra thickness and strength near the end) and threaded. Currently, the magnetic particle inspection method is the primary method used for such end area inspections. In this method, the area to be inspected is magnetized and then dusted with fine particles of iron or iron oxide. These particles accumulate in the presence of cracks, thus giving a visual indication of the location of any flaws. This method is limited in that the area to be inspected must be thoroughly cleaned before the inspection. The method is also dependent on the alertness and visual acuity of the inspector.
A better method for detecting flaws at the end areas of tubular products makes use of magnetically sensitive transducers to detect flaws. A system of this type is described in U.S. patent application Ser. No. 308,749, filed on Oct. 5, 1981 by Moyer et al., the entirety of which is incorporated by reference. The apparatus described therein comprises an apparatus for applying a magnetic force to produce a generally axial field through the threaded end of a pipe and having various means for sensing radial magnetic fields and generating signals corresponding to those magnetic fields. Substantially transverse flaws in the pipe cause discontinuities in the magnetic flux lines which are generated in the pipe. These discontinuities in the magnetic field are detected by the sensing elements. The apparatus is not designed to detect the substantially longitudinal flaws often found in tubing and casing.
Another apparatus for detecting flaws on the ends of tubular products is disclosed in U.S. Pat. No. 3,710,236, issued Jan. 9, 1973 to H. P. Halsey et al. Halsey et al. disclose an apparatus for detecting longitudinal flaws in a magnetically energized body using a pair of overlaying, overlapping Hall devices. The apparatus includes a mounting assembly for fixedly positioning the Hall devices at the proper angle relative to the pipe. Should the body being inspected have both straight and tapered surfaces, the device mounting must be readjusted for inspection of the different surfaces. This apparatus provides no means for mechanically engaging the body being inspected and automatically moving the Hall devices about the end area of the body so as to scan the entire end area.